Week 3 Lecture
October 2001
Metabolism Continued
Lecture Review

Metabolism Basics

Aerobic Metabolism of Organics
This Week’s Lecture

Anaerobic Respiration





nitrate (NO3-), CO2, sulfate (SO42-), ferric
iron (Fe3+), organics, and others
Fermentation
Syntrophic Association During
Conversion of Mixed Acid Products to
Methane
Chemolithotrophy
Photosynthesis
Aerobic Respiration Overview



carbon flows to carbon dioxide
electrons flow to external acceptor
energy produced by oxidative
phosphorylation through PMF
Respiration of Glucose
glucose
ADP
ATP
glycolysis
pyruvate
½ O2
GDP
GTP
Citric
Acid
Cycle
CO2
e-
Electron Transport System
Electrons flow in the
form of reduced
dinucleotides (NADH
and FADH)
H20
Question?

What happens when the environment is
anoxic or anaerobic?

What is the difference between anaerobic
and anoxic?

What impact does this have on organic
carbon biodegradation?

What is the significance of these changes
in environmental management and
design?
Anaerobic Respiration

Some bacteria are capable of aerobic
respiration and anaerobic respiration
(aerobic is preferred due to more favorable
energy production)

Other bacteria that carry out anaerobic
respiration are obligate anaerobes

In either case, the electron acceptor
chosen is based on maximizing free
energy production for cell growth
Anaerobic (Anoxic) Respiration of
Organics

Organic compounds are most often the
original electron donor

Most electron acceptors are inorganics

Electron transport systems in anaerobic
respiration is similar to that of aerobic
metabolism
Examples of Anoxic Respiration
-0.50
increasing energy production
NAD+/NADH2
So/HSCO2/CH4

SO4/S2Eo’
Fumarate/Succinate


NO3- /NO2Fe3+/Fe2+
+ 0.90
½ O2/H20
terminal electron
acceptors other
than oxygen used
less energy
produced
carbon flow the
same as in aerobic
respiration
Nitrate Reduction
(Denitrification)

Conversion of nitrate (NO3-) as an electron
acceptor to ammonia (NH4+) or nitrate (NO2-)

Nitrite undergoes further reduction to produce
nitric oxide (NO), nitrous oxide (N2O), and
nitrogen gas (N2), all of which are lost to the
atmosphere

Denitrification results in a loss of nitrogen from
ecosystems and is only carried out biologically by
bacteria

Nitrogen removal treatment processes
incorporate denitrification
Aerobic Respiration and Denitrification

During
aerobic
respiration,
three areas
where H+ is
pumped out
to establish
PMF
Denitrification


Only two areas
in ETC that
pump out H+ as
compared to
three for aerobic
respiration
Less energy
generated
Methanogenesis and Acetogenesis
CO2 as an electron acceptor
CO2
methanogenesis
CH4
H2
acetogenesis
acetate
Sulfate Reduction



sulfate (SO4)reduction to sulfide (S2-)requires
eight electrons
the first intermediate in this process is the
production of sulfite (SO32-) and requires two
electrons
conversion of sulfite to sulfide requires an
additional six electrons
Sulfate Dissimilatory Reduction
Why does sulfate inhibit methane
formation?

Hydrogen is needed for both processes

Sulfate/sulfide (SO4/S2-) redox pair has a
more positive reduction potential

How would sulfate presence in an
anaerobic digester affect methane
formation?
Iron Reduction





Ferric iron (Fe3+) reduction to ferrous iron
(Fe2+)
Relatively large positive Eo’ indicates that
Fe3+ is an attractive electron acceptor
Ferrous iron is much more soluble and this
process has been used in mining iron ore
Because of the high concentrations of iron in
some groundwaters, iron reduction is a
common reaction in groundwater remediation
Very little Fe3+ in surface waters at neutral pH
Other Metals as Electron Acceptors

Mn+4 to Mn+2


Cr+6 to Cr+3


Cr+3 much less toxic and soluble and is
precipitated out
AsO43- to AsO33

important in drinking water and groundwater
systems
mining wastes
SeO42- to SeO42
major problem in agriculture lands in California
If there are no external electron
acceptors??




Suppose there are no electron acceptors like
nitrate, various metals, etc.
What happens to the electrons associated
with the organic carbon that is oxidized?
How do cells handle this condition?
What is this called?
Fermentation





organic compounds serve as both e donor
and acceptor
no externally supplied e donor
oxidized and reduced products formed
carbohydrates are primary fermentable
substrates
ATP production occurs via substrate level
phosphorylation
Fermentation

Fermentation reactions are important in:
 wastewater treatment processes
 phosphorus removal
 sludge digestion
 BOD removal
 wetland systems, especially in bottom
 sediments (PCB dechlorination)
 agricultural management plans for manure
 landfill leachate management
Fermentation Carbon and Energy Flow
Organic Compound
e donor
P
substrate
level
phosphorylation
ADP
ATP
intermediate
intermediate ~P
electron
carrier
Oxidized Organic
intermediate e acceptor
Reduced Organic
fermentation product
Pyruvic Acid Fermentation
ADP
ATP
organics
pyruvic acid
NAD+
NADH
NADH
NAD+
acetaldehyde + CO2
NADH
lactic acid
NAD+
ethanol
mixed acids
Mixed Acid Fermentation
Complex Organics
propionic acid
pyruvic acid
acetic acid
butyric acid
formic acid
CO2 H2
Mixed Acid Fermentation

important in the breakdown of organic
compounds in anaerobic environments

primary products are organic acids,
carbon dioxide, and hydrogen
Conversion of Mixed Acid
Fermentation Products to Methane

acetic acid and carbon dioxide are
converted to methane in anaerobic
environments

hydrogen is consumed in the process

butyric and propionic acid are not
converted directly to methane
Methane Formation
propionic acid
pyruvic acid
acetic acid
CH4
butyric acid
formic acid
CO2 H2
Methane Formation
propionic acid
pyruvic acid
butyric acid
DGo’ +
DGo’ +
acetic acid
CH4
formic acid
CO2 H2
Mixed Acid Conversion to Acetic Acid

Breakdown of acids such as butyric and
propionic to acetic is required prior to
methane formation

This breakdown is energetically nonfavorable at standard conditions

How do organisms alter the
environment to achieve this reaction?
Non-Standard Conditions
DG = DGo’ + RT ln ([C][D]/[A][B])



Conversion of butyric and propionic acids
results in acetic acid and H2
H2 is consumed by methanogens in the
conversion of both acetic acid and CO2 to
methane
The reduction in H2 makes these reactions
possible by lowering the product
concentrations in the above equation
Syntrophic Association

Where a H2 producing organism can only
grow in the presence of a H2 consuming
organism

The coupling of H2 formation and use is
called interspecies hydrogen transfer

If H2 builds up in a process it is indicative
of an unbalanced consortium

A H2 build-up will result in a build up of
acids resulting in pH decreases and
process failure
Fermentation Summary

Little free energy available for growth




for example in glucose fermentation to ethanol, 2
moles of ATP produced/mole of glucose
Most energy is tied up as products (alcohols,
acids, methane, H2)
These products produced as intermediate
electron acceptors are reduced
A key intermediate is pyruvate
Can other substances besides
organic carbon serve as electron
donors?
Chemolithotrophy

the oxidation of inorganics for production
of cellular energy

terminal electron acceptor is typically
oxygen

most lithotrophs are also autotrophs

accordingly, during lithotrophy there is a
need to not only produce energy in the
from of ATP but also reduced electron
carriers to reduce CO2 to cell carbon
Electron Flow in Lithotrophs
CH2O
CO2
NADP+
e- donor
oxid e- donor
e-
e-
e-
Electron Transport Chain
NADPH
ADP
ATP

O2
energy gained from e- flow through ETC is
used to drive reverse electron transport
against an unfavorable reduction potential to
form NADPH and then reduce CO2
H20
Electron Donors for Chemolithotrophy
-0.50
2H+/H2
S0/HSSO42-/HS-
0.0
NH2OH/NH4+
Eo’
NO3- /NO2-

the greater the
reduction potential
differences between
the donor and
oxygen, the greater
the energy available
for growth
Fe3+/Fe2+
+ 1.00
½ O2/H20
primary electron
acceptor for lithotrophy
Hydrogen Oxidation

chemolithotrophs use hydrogen as an energy
source for growth

those bacteria that use hydrogen as an electron
donor and oxygen as a terminal acceptor are
referred to as hydrogen bacteria (versus
methanogens)

Typically these bacteria are autotrophs that convert
carbon dioxide to cell carbon via the Calvin cycle.
The energy for this comes from oxidation of
hydrogen using oxygen as an electron acceptor
6H2 + 2O2 + CO2
CH20
+ H2 0
Sulfur Oxidation

Oxidation of hydrogen sulfide (H2S),
elemental sulfur (So) and thiosulfate
(S2O32-)

Final Product is sulfate (SO42-)

Very important in acid mine drainage,
biological corrosion
Sulfur Oxidation Reactions
H2S + 2O2
So + H20 + 1/2O2
S2O32- + H20 + 2O2
HS- + 1/2O2 + H+
SO42- + H+
SO42- + 2H+
2SO42- + 2H+
So + H20
sulfur storage as
granules
Iron Oxidizing Bacteria



At neutral pH and ambient conditions, ferrous
iron (Fe2+) oxidizes quickly to ferric iron (Fe3+)
Under acid conditions this reaction does not
occur spontaneously
Lithotrophs (Iron bacteria )biologically
convert Fe2+ to Fe3+ under these conditions
4Fe2+ + O2 + 4 H+

4 Fe3+ + 2 H20
Oxidation of iron results in little energy
production because reduction potential of to
Fe3+ /Fe2+ is so close to that of oxygen/water
Nitrification

Conversion of ammonium (NH4+) to nitrate
(NO3-)

Nitrite (NO2-) is an intermediate

Nitrification is a very important process
agriculturally as it leads to the oxidation of
ammonia to nitrate and potential nitrogen
loss through denitrificiation

In wastewater treatment, nitrification is
often needed to reduce the oxygen
demand the effluent
Nitrification Reactions
Nitrosomonas
NH4+ + 3/2O2
NO2- + H20 + 2H+
Nitrobacter
NO2- + 1/2O2
NO3-

Oxidation of ammonia results in production of acidic
conditions

Very little energy available to nitrifiers because
reduction potential relatively close to that of oxygen
Energy Production During
Phototrophy





the ability to photosynthesize is based on light
sensitive pigments called chlorophylls
all cells have chlorophyll A and typically some others
photosynthesis converts light energy to chemical
energy
chemical energy produced is used for cell growth in
phototrophs which typically are autotrophs (energy is
required to reduce CO2 to cellular carbon)
photosynthesis occurs in both anaerobic and aerobic
environments
Anoxygenic Photosynthesis

energy production in anoxygenic
photosynthesis occurs as a result of
electron flow through an electron transport
chain

electron flow is cyclic

membrane mediated

process is similar to that in aerobic
respiration and some electron transport
components are common to both systems
Electron Flow and Energy Production
in Anoxygenic Photosynthesis
P870
Center
act.
eElectron Transport Chain
-0.60
Eo’
e-
+ 0.40
Antenna
pigment
complex
P870
Center
e-
PMF
ADP
ATP
Schematic of Anoxygenic
Photosynthesis
Photoautotrophs

Phototrophs tend to be autotrophs

As such, there is a need to reduce inorganic
carbon (CO2) to organic carbon
reducing
power
CO2
CH2O

To reduce CO2 to organic carbon takes
reducing power (NADPH)

Autotrophs use reverse electron transport to
produce NADPH
Use of External Electron Acceptors in
Anoxygenic Photosynthesis
Electron Transport Chain
P870
Center
act.
e-
Antenna
pigment
complex
P870
Center
NADP+
e-
e-
ePMF
NADPH
ADP
ATP
e-
H2S
Oxygenic Photosynthesis






involves two distinct but interconnected
photoreaction centers
electron flow is non-cyclic
water is the primary electron donor
as water is oxidized, oxygen is liberated
in Eukaryotic organisms oxygenic
photosynthesis occurs in chloroplast
membranes
in Prokaryotic organism, oxygenic
photosynthesis occurs in cytoplasmic
membrane
Oxygenic Photosynthesis
NonCyclic Electron Flow
-1.0
NADP+
e-
e-
NADPH
Eo’
e+ 0.8
H20
PMF
1/2O2 and 2H+
ATP
ADP
Phototrophy Summary



Use of light energy to produce chemical
energy in the form of ATP and reduced
electron carriers
ATP is used for normal cellular functions and
to reverse normal electron flow where
necessary to produce reduced electron
carriers
reduced electron carriers are used to reduce
CO2 to organic carbon (Calvin Cycle)
NADPH
CO2
NADP+
CH2O